This invention relates to expanded cellular polyolefin products.
One common method of preparing expanded cellular products is to mix a molten polymeric resin with a physical blowing agent in a zone of high pressure and to extrude the mixture into a zone of lower pressure where the blowing agent expands. Foams of low density and having a wide range of uses can be economically produced at high extrusion rates using physical blowing agents.
However, foams produced with physical blowing agents generally do not have the fine cell structure, resiliency, and softness that can be achieved with chemical blowing agents, which tends to limit the market for extruded foams. High quality shoe soles, padded surfaces for exercise equipment, and protective padding can be made from foams prepared with chemical blowing agents.
Foams that are prepared with chemical blowing agents normally are prepared in a two-stage process that is more troublesome and expensive than extrusion foaming. Chemical blowing agents generally are not activated until after the extrusion so that the extruded resin can be cross linked. Chemical blowing agents release a high rate of gas, normally an inert gas, including nitrogen or carbon dioxide. In the absence of cross linking, a chemical blowing agent generally would produce a large number of open, coarse cells.
Foams can be cross linked by irradiation or by free-radical catalysts, including peroxides. In one method, a molten polymer resin is mixed with peroxide and a chemical blowing agent. The temperature is kept as low as possible to avoid premature cross linking. Premature cross linking makes the resin more difficult to extrude and increases the amount of heat generated. Extreme heat tends to produce foams that are unstable and are subject to collapse. The resin is extruded and the extrudate is heated to initiate cross linking and to activate the chemical blowing agent to produce a foam.
Cheng-Shiang U.S. Pat. No. 4,738,810 describes a method of preparing a foam with a chemical blowing agent. The foam is prepared from linear low density polyethylene. Excessive cross linking is said to be precluded by premixing a chemical bowing agent, a cross linking agent, and other polymeric components before mixing with the linear low density resin.
Ionomer is said to be useful as one of the other polymeric components. Ionomers are copolymers having an ionizable comonomer. Ionomers are normally prepared by copolymerization of ethylene with small amounts of an unsaturated carboxylic acid, followed by ionization of the acid group to yield a metal salt. The ionized groups act as meltable cross links. Ionomers have been used to improve the toughness and opacity to films, including the multilayer films that are used in vacuum skin packaging. Ionomers have also been used in resins for extrusion foaming under certain circumstances.
For example, Watanabe et al. U.S. Pat. No. 4,102,829 discloses low density extruded foams prepared from a mixture of from about 5 to 65% polyolefin and from about 35 to 95% ionomer. The foams are said to have a good balance of properties, including thermal resistance, and are indicated to be useful as an insulation covering on pipes for an air conditioner.
O""Brien et al. U.S. Pat. No. 4,091,136 discloses a synthetic cork-like material for use as a closure for liquid containers that is composed of an extruded fine-celled polyolefin foam containing an ionomer. The O""Brien et al. patent describes preparing a polyethylene foam rod with from 0.5 to 35 weight percent of DuPont Surlyn ionomer in the foamable resin mixture. The presence of ionomer is said to provide sufficient structural strength to the polyolefin foam so that it can be handled in conventional corking equipment.
Cylindrical product profiles that characterize insulation covers for air conditioning pipes and synthetic cork generally result in relatively low shear in the extrusion process, on the order of 10 secxe2x88x921. Shear generates heat, which reduces melt strength and can be problematic, particularly at higher levels, resulting in unstable foams that tend to collapse.
Extrusion foaming of sheet product profiles generally results in higher shear in the extrusion process of about 100 secxe2x88x921 or more. Shear can be several orders of magnitude greater for the production of sheet than for cylindrical product profiles.
High shear generation means that the heat generated by extrusion can be problematic. The xe2x80x9cprocessing windowxe2x80x9d of suitable operating parameters of shear, melt temperature, and extrusion throughput for producing foam sheet products is relatively narrow compared to cylindrical product profiles. The process of extrusion foaming of sheet products normally will not tolerate cross linking in the resin, particularly at higher shear rates. Cross linking can render a resin unprocessable, particularly at high shear. Accordingly, there is not believed to have been any disclosure or suggestion to incorporate ionomer into polyolefin resins for extrusion as sheet or plank.
It would be desirable if polyolefin foam sheet products could be prepared with the economies of the extrusion foaming process that could be competitive with foams prepared from chemical blowing agents. However, polyolefins are relatively low modulus polymers that normally do not have the melt strength to form extruded foams of the fine cell structure and resiliency that is achieved with cross linking and expansion with chemical blowing agents.
Bambara et al. U.S. Pat. No. 5,876,813 discloses preparing an extrusion foamed polyolefin foam structure that is said to have enhanced properties by laminating a high density skin to a low density core. The high density skin is said to improve the flexural stiffness and wear properties of the foam structure. The structure is said to be useful in collapsible packaging systems because the structure can be die cut so that the higher density skin can act as a hinge allowing the die cut piece to be folded.
The invention relates to the use of ionomers in polyolefin resins for the production of polyolefin foam sheet having improved properties, and in particular, to extrusion foamed polyolefin foam sheet that is useful in collapsible packaging systems, whether in a laminated structure or not. In laminated structures, the skin can be the same or nearly the same density as the core. Whether in a laminated structure or not, the skin is of suitable tear strength and enhanced other properties to act as a hinge for die cut collapsible foams. It should be understood that, as used herein, the term sheet designates thin polyolefin extruded foams of less than about xc2xd-inch in thickness and also includes the thicker plank product profiles of up to about 3 inches or so.
It has been determined that polyolefin foam sheet can be produced by a single stage extrusion foaming process having an acceptable processing window when ionomer is incorporated into the resin at particular levels. The foam can be made recyclable, which generally is not true of foams prepared from chemical blowing agents. Foam sheet products having improved properties, including tear strength, thermoformability, cushioning, creep resistance, compression strength, hysteresis, and a softer touch, can be achieved at the high extrusion foaming throughputs that are desired commercially. High throughput can be achieved on existing single-stage extrusion equipment without having to make special or difficult process adjustments.
The product of the invention, and, in particular, the skin that is formed on the surface of the product as a result of extrusion foaming, has tear strength sufficient for use as a hinge or joint between foam bodies for collapsible packaging systems. The foam of the invention can be cut through one surface and the core thereof to the other surface without cutting through the other surface to create the hinge. The foam sheeting of the invention can be laminated to other structures, which may or may not be manufactured according to the invention, and used as a hinge.
At typical shear conditions for sheet products, the ionomer should be present in the polyolefin resin in an amount of from about 1 to 25% by weight, based on the polyolefin and ionomer components, for most applications. The extruded foam sheet product will have a similar amount of ionomer. Above about 25%, the processing window becomes narrow and it is more difficult to produce an acceptable product. However, at relatively lower shear, it should be possible to use up to about 40% by weight ionomer in the resin. The temperature of the melt at the exit of the extruder should be maintained at less than or equal to about 238 degrees Fahrenheit, and usually at about 230 degrees.
The density of the foam can range from about 20 to 150 kilograms per cubic meter. For many packaging and thermoforming applications, including disposable trays for medical and dental instruments and the like, the density is normally from about 20 to 50 kilograms per cubic meter. The foam can be prepared with fine cells of from about 15 to 60 cells per square inch. The foam can generally successfully resist an impact of from 200 to 425 pounds per square inch. The foam can be produced as thin sheets of about one-half inch or less in thickness. Thicker plank foam products can be produced by extruding the foam in sheets of one-half inch or greater thickness, up to about 2 inches, and by laminating two or more thin sheets to a desired thickness of up to about 3 inches.
The resin mixture from which the foams are made can optionally include a metallocene polyolefin, which normally is a metallocene polyethylene, and which further strengthens the resin for expansion with a blowing agent. The metallocene generally is present in an amount of from about 5 to 30% by weight of the resin. The density of the resin should normally be maintained at or below about 0.930 g/cm3 for ease of single step extrusion foaming.
Thus, the invention provides, among other benefits, an expanded cellular polyethylene sheet product of low density having improved characteristics that are competitive with foams prepared from chemical blowing agents. The resin from which the product is made has processability suitable for single stage expansion. The expanded product is recyclable and has improved tear strength, thermoformability, and packaging characteristics without burdensome adjustments in processing conditions. Die cut collapsible foam packaging systems can be produced. While not wishing to be bound by theory, it is believed that the ionomer develops a reversible physical crosslinking within the foam that provides the improved strength properties. The bonds are thermally reversible, which provides recyclability.